[FLINK-23458][docs] Added the network buffer documentation along wit…

[akalash]

…h the buffer debloat doc

What is the purpose of the change

The documentation for network buffer along with buffer debloat

Brief change log

• Added the documentation of the network buffer

Verifying this change

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Documentation

• Does this pull request introduce a new feature? (yes / no)
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[pnowojski]

Can you add a section about manually configuring the amount of in-flight buffers? That for max throughput we recommend the default settings of exclusive buffers and floating buffers. For manually minimising the amount of the in-flight data the exclusive buffers can be set to 0 and the memory segment size can be decreased.

Additionally I think we don't have any warning/info message logged if not all configured amount of buffers was acquired. I think we should add this on the warning level. Especially after FLINK-16641, if we fail to allocate even a single exclusive buffer on the input gates, the performance would suffer dramatically and I think at the moment users would have no way of knowing that this has happened.

[pnowojski]

Also as we discussed off line, I think the best place for this docs is a sibling of this page: https://ci.apache.org/projects/flink/flink-docs-master/docs/deployment/memory/mem_tuning/

And can you add cross references from the configuration and memory configuration docs to this networ memory tuning page?

[pnowojski]

One last comment from my side. I would ask Daisy to take a look, as I'm pretty sure there are hundreds of linguistic errors that we can not catch (like missing the, an, a 🙄 )

[akalash]

@infoverload, thanks for the suggestions. I have applied them all. If you have more comments I will wait for them if not, please, approve the PR if it looks good to you.

[akalash]

It looks like everybody has finished with the review of this PR so if there are no objections I will squash commits and then we can merge it. @pnowojski , @infoverload

[pnowojski]

Yes, please squash the commits.